Sharp JX-9400 Información técnica descargar pdf (Pagina 33)

The air mass conservation (Equat ion 1.23) is rewritten as:

¼ T

j ¼0

ð1  



j ¼1

ð1  

Þþ

ð1:43Þ

These last two systems include N þ 1 equations for N þ 1 unknowns, q

, for

each zone i.

It should be noticed that Equation 1.43 can be simpliﬁed, and becomes

similar to Equation 1.23 if indoor and outdoor temperatures are close to each

other and if the internal temperature is constant. This means that, provided

such conditions are realized, the volume conservation equation can be used

instead of mass conservation.

Summary of the various tracer gas methods

The diﬀerent tracer gas techniques can be broadly divided into two categories:

steady-state methods, which directly measure the ﬂow rate, Q, and transient

methods, which measure the nominal time constant, 

, or the air change rate,

n. The steady-state techniques are based on recording steady-state conce ntra-

tions or concentrations integrated over a long time, while transient methods are

based on recording the change in tracer gas concentration. The di ﬀerent tracer

gas techniques and their properties are given in Tables 1.1 and 1.2.

If airﬂow varies with time, only the two-point decay and the constant

concentration methods give a correct estimate of the average ﬂow. The constant

injection meth od underestimates the average ﬂow rate if the integration time is

much longer than the period of ﬂow variation.

Table 1.3 gives a summary of multi-zone methods. As far as single-zone

measurements are concerned, the following conclusions can be stated:

It appears that decay, pulse and step-up methods require the least measure-

ment time and usually the least preparation. However, with the exception of

the two-point decay method, they give a biased estimate of a variable air

change rate. These biases remain small if the measurement period is limited

to times close to the nominal time constant.

Table 1.1 Summary of diﬀerent injection strategie s

Tracer injection strategy Direct result Cost

Pulse injection Q

Moderate

Decay n or 

Moderate

Constant injection rate Q

Moderate

Constant concentration Q Relatively high

Note: y The volume has no inﬂuence only when the airﬂow rate, Q, is constant.

12 Ventilation and Airﬂow in Buildings

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